1. Field of the Invention
The present invention relates to an organic EL display device and a manufacturing method of the same.
2. Description of the Background Art
In recent years, an organic EL (Electro Luminescence) display device including an organic EL element has increasingly been developed. As the organic EL display device is of a self-emitting type, a backlight as required in a liquid crystal display device is not necessary. In addition, as smaller thickness and lighter weight can be achieved and an angle of view is wide, the organic EL display device is expected to serve as a next-generation display. In particular, an active-matrix organic EL display device in which a thin film transistor (TFT) is arranged for each pixel as a drive element for driving an organic EL element has attracted attention.
A structure for extracting light that is emitted from a light-emitting layer includes a structure for extracting emitted light from the bottom side (hereinafter, referred to as “bottom emission structure”) and a structure for extracting emitted light from the top side (hereinafter, referred to as “top emission structure”), with respect to a glass substrate on which a TFT or the like is formed. The top emission structure is capable of achieving higher definition and improved light extraction efficiency, because light is not blocked by the TFT.
An organic EL element has such a structure that an organic EL layer is sandwiched between an anode and a cathode. The organic EL layer may be implemented only by a light-emitting layer or by a multi-layer structure formed by layering a light-emitting layer, a hole transport layer and an electron injection layer. The organic EL element emits light, for example in such a manner that, by applying a voltage across the anode and the cathode, electrons injected from the cathode through the electron injection layer and holes injected from the anode through the hole transport layer are recombined within the organic EL layer. Meanwhile, the organic EL layer is composed of a low-molecular type organic EL material, a macromolecular type (polymeric) organic EL material, and the like. The low-molecular type organic EL material is formed with a vapor deposition method or the like. The macromolecular type organic EL material is formed with a coating method using spin coating or an ink jet method.
The organic EL element has such a disadvantage that deterioration thereof proceeds due to moisture, which leads to lower luminance and poor long-term reliability. As characteristics of the organic EL element are readily impaired by moisture, reaching of moisture to the organic EL element is preferably suppressed as much as possible. Moisture reaching the organic EL element is broadly categorized into moisture that is introduced from the outside of the display device and moisture contained in the display device.
Example of moisture that is introduced from the outside of the display device includes moisture contained in normal air in which the organic EL display device is used. The moisture passes through a pin hole or the like of the cathode in an uppermost layer that forms the organic EL element, and diffuses in the inside to cause, for example, such a defect that a non-emitting region called a dark spot grows in a circular shape.
Introduction of moisture from the outside of the display device is prevented by covering the entire surface of the organic EL element with a covering film such as a barrier layer formed with an inorganic insulating film composed of SiNx, SiOxNy or the like, a resin film, and a film implemented by layering these films. Alternatively, all of a plurality of organic EL elements are covered with a cover glass and then a peripheral portion of the cover glass is sealed with a photo-curing adhesive such as an epoxy resin, thus preventing introduction of moisture. Sealing with such a covering film and a cover glass is performed in a vacuum atmosphere or in a dry inert gas atmosphere having a dew point around −80° C.
Alternatively, a getter material or a getter film containing CaO, BaO, SrO, or the like is arranged in a space covered with a cover glass or the like. By arranging such a hygroscopic member, moisture that is introduced from the outside is absorbed, so that reaching of moisture to the organic EL element is suppressed.
An example of moisture contained in the display device includes moisture contained in an organic insulating film. Moisture present in a flattening film formed between the organic EL element and a surface where a TFT is formed and implemented by an organic insulating film represents one example. In addition, moisture present in an isolation film arranged to surround a pixel region where the organic EL layer is arranged and implemented by an organic insulating film represents one example. The isolation film is also referred to as a pixel isolation film, and it is formed, for example, to cover an end portion of an anode and a region where an anode is not present.
The organic insulating film may contain a photosensitive resin. The organic insulating film is formed, for example, by arranging a photosensitive resin with spin coating, thereafter performing exposure to form a desired pattern using a photomask, and thereafter performing a development treatment. After the development treatment, heat treatment is performed in atmosphere. The heat treatment is referred to as a curing and baking treatment or a post-baking treatment. By performing the curing and baking treatment, a coating solvent contained in the organic insulating film is removed and the insulating film is burnt. In the curing and baking treatment, for example, heating at a temperature of approximately 200° C. or higher and approximately 300° C. or lower is performed for 0.5 to 1 hour, although there is some difference depending on an organic material.
Japanese Patent Laying-Open No. 2003-332058 discloses a method of manufacturing an electroluminescence panel including the steps of: forming a first electrode on a substrate; forming an insulating layer around the first electrode; subjecting the insulating layer to heat treatment so as to lower an amount of moisture contained in the insulating layer; forming an electroluminescence layer on the insulating layer; and forming a second electrode on the electroluminescence layer.
Moisture contained in the organic EL display device diffuses to move toward the organic EL element. When the moisture reaches the organic EL element, such a defect that a dark region where emission intensity is low grows from a peripheral portion toward the center of a pixel appears. Thus, display characteristics of the organic EL display device are lowered due to moisture present in the display device. The moisture reaches the organic EL layer mainly through the flattening film or the isolation film as the diffusion path.
In general, the flattening film or the isolation film is implemented by an organic insulating film. As a material, the organic insulating film is likely to absorb moisture. Accordingly, the organic insulating film absorbs moisture in such steps as a wet treatment or a washing treatment with water in the manufacturing process. Alternatively, the organic insulating film absorbs a large amount of moisture during transportation through atmosphere.
Japanese Patent Laying-Open No. 2003-332058 above discloses heat treatment of the insulating layer for 1 hour to 3 hours at a temperature from 200° C. to 270° C. With the heat treatment, however, it has been difficult to sufficiently obtain stability of luminance and long-term reliability. Though an allowable upper temperature limit of the organic insulating film depends on a material for the same, an allowable upper temperature limit of a general organic insulating film is not lower than 200° C. and not higher than 300° C. For example, after continued use of a product while being heated for 3 hours at the allowable upper temperature limit, a dark region from an outer peripheral portion of a pixel was observed in several hundred hours.
In particular, an active-matrix organic EL display device having a TFT as a drive element for driving a pixel has such a complicated structure that a TFT, a capacitance portion, an interconnection, and the like are present under a pixel electrode and these elements are covered with a flattening film. Here, simply performing heat treatment described in the publication above has been insufficient to remove the contained moisture. Though a dehydration effect is enhanced by extending a time for dehydration, productivity in manufacturing the organic EL display device becomes poorer (poorer throughput). Therefore, it is not allowed to perform dehydration for a long time and a major problem of reliability has still remained.
In the step of manufacturing a flattening insulating film or an isolation film, a curing and baking step of performing heat treatment as a step of solidifying an organic insulating film after the organic insulating film is formed is present. For removal of moisture, a dehydration step using heat treatment is preferably performed other than the curing and baking step, however, a temperature and a time period for heating are restricted. Therefore, in a range of time ensuring throughput, it has been impossible to perform the dehydration step sufficient for suppressing deterioration of the organic EL element.